The prosthesis is of the type, as described for example in applicant's U.S. Pat. No. 4,001,897, with two parts forming a ball-and-socket joint. One part, female, is designed to replace the hip socket and the other, male, is designed to replace the head of the femur. Each of the two elements of this type of prosthesis is composed of two parts, a base attached irremovably to the bone and an active organ attached removably to the base. The active organ is composed of either a male or female sphere.
According to a particular embodiment of the prosthesis, as described in applicant's U.S. Pat. No. 4,001,897, the base of the male element is a plate designed to bear on the upper end of the femur, the bearing face of which has a rod designed to engage the femur, and on the other face of which is mounted a split collar equipped with a tightening bolt and having a threaded bore and a smooth bore. The active organ of this male element is composed of a sphere provided with a cylindrical radial projection having a threaded zone designed to engage the threaded bore of the split collar of the base and a smooth part designed to tighten it in the collar.
Attachment of the active organ to the base of this male element is thus provided by screwing the threaded part of the radial projection of the active element into the threaded bore of the base and by tightening of the split collar by means of a tightening bolt which ensures that the two elements are rotationally immovable with respect to each other.
It will readily be understood that for effective locking it is necessary for the collar to exert a powerful tightening force on the smooth part of the radial projection of the active element. To obtain this powerful tightening force, it is necessary to have a bolt able to withstand very large twisting and pulling stresses without risk of breaking, so that bolts with a large cross section are used. As a result, the collar must be oversized to accommodate the tightening bolt, leading to relatively large male element bases.